Epidemiological studies on accidents (e.g. bicycle accidents) show that a substantial number of the subjects who call for medical aid, are suffering from skull and brain damage. Furthermore, cranio-cerebral traumas are a direct cause for the majority of the fatal accidents. A protection helmet should therefore protect the head against these traumas.
There are many types of protective helmets on the market, with different designs and characteristics. They are designed to satisfy legal requirements, but do generally not offer a protection to the most common skull and brain damages. At present, these legal requirements are related to the maximum linear acceleration that may occur in the centre of gravity of the brain at a specified load, and may involve tests in which a so-called “dummy skull”, equipped with a helmet, is subjected to impact. As a result of these legal requirements, helmets that are currently available on the market offer a good protection in the case of a normal impact on the head. Fractures of the skull and/or pressure or abrasion injuries of the brain tissue typically occur after this type of impact. These helmets generally consist of three functional units, which are conceived in three separate layers that are always ordered as follows: a hard outer shell that distributes forces acting on the head over a larger surface, an energy-absorbing middle shell, and an inner layer that guarantees a comfortable fit on the head.
However, mathematical simulations (see FIG. 1) show that rotational accelerations of the head increase with an increasing tangential component Ft of the impact force F (see FIG. 3), while helmets that are currently available on the market do not offer a sufficient protection against impact that is tangential to the head. Furthermore, literature (both early and recent [1]-[7]) shows that the most common brain injuries are related to rotational accelerations (not linear accelerations) while legal requirements and standards do not include this aspect. Typical injuries related to head rotation are contusions, ASDH (Acute Sub-Dural Haematoma; bleeding as a consequence of blood vessels rupturing), and DAI (Diffuse Axonal Injuries; widespread damage to axons in the white matter of the brain). Although the understanding of the precise mechanical processes that lead to these specific injuries is still imperfect, recent research [7] has revealed, inter alia, a relation between brain parenchyma and bridging vein lesions on the one hand and the rotational acceleration of the head on the other hand. The type and the severity of the injury depend on the development of impact parameters as a function of time, such as the duration and the amplitude.
US 2002/0023291 A1 describes a helmet designed to protect the head and brain from both linear and rotational impact energy, constructed of 4 layers, the layers comprising polyurethane, monoprene gel, polyethylene and either polycarbonate or polypropoylene. U.S. Pat. No. 6,658,671 describes a protective helmet with an inner and an outer shell with in between a sliding layer and whereby the inner and the outer shell are interconnected with connecting members. EP1142495 A1 describes a helmet in which a layer of elastic body (which may be a gel) is provided between the inner side of the shell and the shock absorbing liner, or in between two layers of the shock absorbing liner. WO2004/032659A1 describes a head protective device with an inner and an outer layer, and an interface layer with a spherical curvature, allowing displacement of the outer layer with respect to the inner layer. The interface layer may consist of a viscous medium, a hyper-elastic structure, an elastomer-based lamellar structure, or connecting members. These helmets, however, only allow a limited rotational displacement of the inner shell with respect to the outer shell, because the shape of the helmet is not a perfect hemisphere. Consequently, the energy that can be dissipated is limited as well. Furthermore, these helmets have poor ventilation capacities, and are relatively complex to manufacture.